How do you deal with HR for notice period?

My organization notice period is 3 months and I want to get relieved in 1 month. But the HR is not allowing me to go for buyout as I need to undergo as dental implant treatment which would take more than 2 months to finish and recover from the treatment. HR is coming up with the statement that the organization doctor has given a different consent. HR is asking me to go on loss of pay for the remaining days if I don't want to work on those days. In the offer letter there is option to buyout notice period in case of medical reason. But the HR is not agreeing for it. What should I do in this case.